IF you do:
apt-get -t unstable install kernel-source-2.6.7
You will get a compressed archive of the kernel source in /usr/src. The Debian source has some patches applied, so it is not exactly the same as the one from www.kernel.org.
The base Debian install doesn't have everything you need to compile. When you run the above command some of the required dependencies will be installed automatically with apt-get. Also look for "recommended" and "suggested" packages when running the above command. You will probably need to add kernel-package to use "make-kpkg" and some ncurses packages if you want to use "make menuconfig".